Process for minimizing dioxin formation during waste and biomass utilization

ABSTRACT

A process for the production of high quality synthetic coal from biomass or urban waste, while effectively reducing its potential for dioxin production. The feedstock is first sorted to remove recyclable inorganic materials. After size reduction, the feedstock is pyrolyzed at a temperature range of 500 to 600° F. (260-315° C.), in a high capacity, continuous mixer reactor, using in-situ viscous shear heating of the waste materials, to produce a highly uniform, granular synthetic fuel product similar in energy content, storage and handling characteristics to, but much cleaner burning than, natural coal. The process effectively destroys dioxins and other chlorinated hydrocarbons that may be present in the feedstock, while removing and sequestering chlorine as alkali salts.

FIELD OF USE

The present invention relates to an improved process for producing a synthetic coal of superior quality from biomass or urban waste, and in the process dramatically reduces the potential for dioxin formation that is prevalent in conventional biomass combustion, incineration and waste-to-energy technologies.

BACKGROUND OF THE INVENTION

The disposal of urban waste has been traditionally handled by landfilling. However, landfilling has become less of a solution to waste disposal and more of a means of storing waste until an effective means of disposal or utilization can be developed. The desire to reduce the amount of waste volume landfilled, and to avoid some of the issues associated with less than perfect waste containment in landfills, has led to programs for recycling, composting and incineration of urban waste materials. Each program brings some benefit, but does not represent a full solution for the waste problem. Effective recycling requires economic justification, and most components of the waste stream do not have sufficient economic value to offset their cost of separation and recovery. Composting is effective on some parts of the waste stream, but the majority of the waste is not amenable to compost production. Incineration converts the organic fraction of the waste stream to heat energy which can be used to generate process steam or electricity. However, the high moisture content, variability of composition and physical characteristics of urban waste have made incineration systems expensive, inefficient, high maintenance, and unpopular with the general public. Worse still, the incomplete combustion of waste and biomass produces toxic dioxins. The U.S. Environmental Protection Agency has determined that nearly 80% of the dioxins being emitted in the United States are the result of the burning of waste materials—backyard refuse burning, the incineration of municipal waste, medical waste, waste water treatment sludge, hazardous and industrial wastes, and the burning of biomass fuels. What is needed is a process that can alter the chemical and physical characteristics of urban waste in such a way as to make its handling, storage, and utilization fully compatible with existing uses, technology and infrastructure, while at the same time minimizing its potential for dioxin formation. By far the most likely and logical end user for the calorific value of the waste stream is the fuel industry.

Many have looked to pyrolysis as a means of chemically altering and improving the combustion characteristics of urban waste and biomass. Heat is added to the waste materials in an oxygen-free environment, breaking the organic matter into a slate of products ranging from mineral matter to carbon-rich charcoal, to oil mixtures, to non-condensible gases and water vapor. Some have sought to focus on the oil byproducts, looking to the motor fuel industry as a potential market. However, waste pyrolysis oils are not readily compatible with petroleum-based liquid fuels, and therefore require extensive and expensive upgrading to achieve that compatibility. Furthermore, the U.S. Environmental Protection Agency has reported that diesel and gas engines contribute over 7 percent of the dioxins that enter our environment.

Other researchers have modified the pyrolysis process to increase production of fuel gases. However, the pyrolysis gases are not compatible with today's natural gas pipeline systems, and must be used on-site. Still others have sought to drive the pyrolysis process to its extreme, yielding a small quantity of concentrated solid carbon. These processes are often characterized by high energy consumption, that is, low thermal efficiency, high reaction temperature, low product yield, long processing time and batch processing. This avenue also requires expensive upgrading, as the char from most pyrolysis processes using urban waste does not have the porosity, surface area and high chemical reactivity desired by the activated carbon market.

Other pyrolysis processes differ in the means by which reaction heat is conveyed to the waste materials, the source of that waste heat, the means by which solids are conveyed within and from the reactor, and the pyrolysis processing conditions themselves early processes used the partial combustion of the solid material to produce high temperature gases that directly contacted and heated the fresh waste material. However, these processes have little temperature control, and produce a wide spectrum of byproducts ranging from tars and heavy oils to light combustible gases, all diluted by the products of partial combustion. While the transfer of heat to the feed material is efficient, the handling of the byproducts is often difficult.

Most pyrolysis processes recognize the desirability of avoiding the heating of the waste by direct contact with hot combustion gases, and have developed a wide range of indirect heating schemes. A few have involved the circulation of hot inert solids from a combustion reactor to the pyrolysis reactor, but most rely on the conduction of heat from hot combustion products or electric heating elements to the waste solids through a heating surface or reactor wall. These designs suffer from several limitations. (1) Pyrolysis heating only occurs for material in direct contact with the heating surface, and is much less effective for the remainder of the material, (2) waste must be well stirred in order that all waste has sufficient contact with the heating surface, (3) for heat to flow, the wall surface must be significantly higher in temperature than the waste material, making the wall surface a target for high temperature corrosion, (4) reactor designs in which the waste containment is interior to the combustion gas containment must be heavily insulated to avoid loss of valuable heat through the exterior walls rather than through the interior heating surface to the waste material, (5) the temperature of the combustion gases leaving the reactor is always higher than the waste temperature and may be higher than the maximum reaction temperature, resulting in low thermal efficiency unless some mechanism is provided for utilization of that heat, (6) high temperature wall surfaces may be prone to overheating of the mineral matter in urban waste, resulting in the production of sticky deposits on wall surfaces, similar to those produced in cement, lime and taconite kilns, (7) systems where only a portion of the waste is subject to heating at one time often result in end products that see a wide range of variability in the amount of pyrolysis that has been achieved, with some material overcooked and other material relatively raw, and (8) these systems are limited by the effectiveness and availability of heating surface.

Most of the proponents of pyrolysis technology have focussed on process technologies that produce a certain mix of fuel products, without consideration for the important dioxin question. Dioxins may be present in some waste products, and can be emitted to the environment if the combustion of those wastes is incomplete, and insufficiently high in temperature to destroy these dioxins. However, another important source of dioxins is through the “de novo” synthesis route. In this case, polyaromatic aromatic hydrocarbons (PAH's) such as naphthalene, phenanthrene and pyrene are formed as a result of incomplete combustion of the waste or biomass. Forest fires, waste incineration and cigarette smoke are examples of sources of these PAH's. In the presence of chlorine and excess oxygen, dioxins and furans can be formed from them. These reactions occur over a fairly narrow temperature range, with the highest formation rate around 650° F. (343° C.).

Numerous researchers have evaluated the conditions needed for formation of PAH's, and have determined that the temperature range for formation is fairly narrow, starting around 1300° F. (700° C.), and reaching a peak around 1,600° F. (870° C.). See FIG. 1. Above about 2,000° F. (1,090° C.), PAH's are destroyed by combustion, but conventional waste incineration technology does not reach the temperatures needed for their complete destruction. Researchers have demonstrated that dioxins are not formed in the reducing environment of low temperature pyrolysis, since neither oxygen nor the PAH precursors to dioxin formation are available.

On the other hand, dioxins can be formed from the PAH's in an oxidizing environment, but the dioxin formation temperature range is also very narrow, maximizing at a temperature of only 650° F. (343° C.). See FIG. 2 wherein A is furans and B is dioxins. Also necessary is the presence of excess oxygen and free chlorine, as would be available in conventional incineration.

Thus, the production of de novo dioxins is essentially a two-step process, first creating the PAH pre-cursors to dioxins, during incomplete combustion at temperatures in the range of 1,600° F. (870° C.), followed by the combining of these PAH's with chlorine and excess oxygen at the much lower temperatures around 650° F. (343° C.) that are present in the cooling incinerator stack gases. The result is the formation of de novo dioxins prevalent in waste and biomass combustion.

The waste and biomass processing technology described here follows a much different reaction and temperature path, effectively reducing the potential for dioxin formation. Numerous researchers have demonstrated that low temperature devolatilization of coal results in the liberation of most of the chlorine at low temperature, with a maximum liberation rate occurring around 550° F. (288° C.). Other researchers have extended these findings to the pyrolysis of biomass, determining that the release of this chlorine, in the form of HCl, is maximized at the same narrow temperature range peaking at about 550° F. (288° C.). In addition, the overall release rate for biomass pyrolysis products is maximized in this same narrow temperature range, indicating that pyrolysis efficiency is highest at around 550° F., and that higher operating temperatures produce deminishing returns. See FIG. 3 wherein A represents chlorine liberation from straw, B represents chlorine liberation from coal, and C represents liberation of all pyrolysis products.

One researcher correlated this liberation of chlorine with the potential to destroy dioxin-contaminated materials. He proposed a process and reactor design in which dioxins were absorbed on the surface of activated carbon, and pyrolyzed at relatively low temperature. However, because of the limitations in internal heat transfer and product mixing present in his vertical moving bed reactor, it was necessary to add alkali agents such as sodium hydroxide or sodium bicarbonate, and to operate the reactor at temperatures above 680° F. (360° C.), well beyond the ideal temperature for chloride release. Nevertheless, dioxin destruction up to 90% was reported. Three possible reaction paths were proposed, involving (1) the stripping of free chlorine molecules from the dioxin molecule, leaving the base oxygenated aromatic hydrocarbon, (2) the stripping of both free chlorine and oxygen from the dioxin molecule, leaving simple aromatic fragments, and (3) the stripping of the oxygen binding the dioxin molecule together, leaving chlorinated aromatic hydrocarbon fragments.

These reaction paths are representative, but not completely consistent with the findings of most others, who reported that the chlorine is released in the form of HCl, not elemental Cl. Others have reported the complete absence of dioxins, PAH's and chlorinated hydrocarbons in bio-oils and chars produced by low temperature pyrolysis.

Researchers at the Technical University of Denmark investigated the fate of chlorine and sulfur released during the low temperature pyrolysis of straw. They determined that roughly half the sulfur and chlorine left the biomass char as H₂S and HCl, while the remainder could be captured by alkali present in the straw ash—particularly potassium and calcium. As the pyrolysis temperature approached 1,000° F. (538° C.), however, competing reactions by ash silica were preferred, tying up the alkali, and preventing chlorine and sulfur capture. At much higher pyrolysis temperatures, the alkali capture reactions were again favored, but the biomass was devolatilizing too quickly for capture to take place. Thus the preferred pyrolysis temperature for maximum chlorine release and capture would be in the range of 550° F. (288° C.). These researchers were proceding with the intent to then wash the alkali-chloride salts from the char, but were unsuccessful in doing so, as the char particles had very limited porosity.

A preferred waste and biomass pyrolysis process for dioxin control must operate in a very narrow range of temperatures, focussing in on a maximum temperature in the range of 550-600° F. (288-316° C.). Below this, the liberation of chlorine is not effective, while above it chlorine capture by alkali is blocked by ash silicates. The destruction of dioxins and other chlorinated hydrocarbons pre-existing in the waste or biomass is maximized in this same narrow temperature range, with the liberation of chlorine. Higher reaction temperatures result in the formation of PAH's, which are then pre-cursors to de novo dioxin formation. Furthermore, dioxins are a relatively trace component of biomass and solid waste, and are scattered throughout the material. If heating and temperature are not uniform, and the material is not well stirred during the reaction process, some portions of the biomass will be subject to higher temperatures and some to lower temperatures, resulting in less than complete dioxin destruction, less alkali capture, and the potential for formation of de novo dioxin precursors. Thus pyrolysis heating systems that rely on contacting the feedstock with hot circulating solids, partial oxidation, heat conduction through external reactor wall surfaces, or radiation to feedstock particle surfaces cannot achieve the degree of temperature and concentration uniformity required to maximize dioxin control. These other systems inevitably result in the need for catalyst and/or alkali addition, long residence times, and limited dioxin destruction efficiency.

Beyond the destruction of dioxins in the waste or biomass feedstock, it is also necessary to transform the waste or biomass into a fuel product that will not itself be prone to de novo dioxin production when burned in a conventional combustion process. The U.S. Environmental Protection Agency has extensively studied the effect of fuel total chloride content in wastes and biomass combustion. They concluded that the key to minimizing dioxin formation is to have a fuel and combustion system that will destroy any PAH precursors before they can cool and form de novo dioxins. Completeness of combustion and high combustion temperature are critical requirements. An ideal fuel will be easily ignited, highly reactive to promote stable, rapid and complete burning, and will produce an ash free of unburned carbon at the conclusion of the combustion process. In addition, the fuel should limit the availability of free chlorine in the combustion products.

Extensive studies have shown that the combustion environment of coal-fired power plants is ideal for dioxin control. Flame temperatures are high, and finely pulverized coal burns rapidly, destroying PAH precursors. These studies found that flyash concentrations of both dioxins and PAH's were near or below laboratory detectability limits. The EPA has catalogued relative dioxin emission factors for a wide range of combustion applications and has concluded that coal power plant dioxin emission factors are lower than those of incinerators by as much as four orders of magnitude, and even an order of magnitude below that for clean woody biomass combustion.

Thus an ideal fuel product would be one that is directly compatible with existing coal power plant boilers, handling systems and combustion technology. This fuel would be very low in moisture content to enhance power plant efficiency; comparable to coal in density, heating value and pulverization characteristics to minimize impacts on fuel unloading, storage, conveying, pulverization and burner systems; and would be highly reactive to insure easy ignition, good flame stability and completeness of combustion. In addition, this fuel would be lower in primary pollutants such as sulfur and mercury, would limit the availability of free chlorine in the combustion products, and reduce the net greenhouse gas emissions of the plant burning it, not just by substitution of “renewable carbon” for “fossil fuel carbon,” but in real tons of total CO2 emitted.

U.S. Pat. No. 6,072,099 issued to Tenaka discloses a process by which dioxins can be absorbed on activated carbon. This material is then pyrolyzed in a moving bed vertical downflow reactor, in the presence of added alkali salts such as sodium hydroxide or sodium bicarbonate. Oxygen levels within the reactor were maintained below 1 percent. When processing times exceeded at least one hour, and reaction temperature exceeded 680° F. (360° C.), dioxin destruction up to 90% was achieved. Tenaka proposed three possible reaction paths, involving (1) the stripping of chlorine from the dioxin molecule, leaving the base oxygenated aromatic hydrocarbon, (2) the stripping of both chlorine and the oxygen that binds the dioxin ring structures together, and (3) the stripping of the oxygen binding the rings, leaving simple chlorinated armomatic hydrocarbons.

U.S. Pat. No. 6,558,644 that issued to Berman describes a process for preparing activated carbon from urban waste. The waste is first sorted to remove foreign materials, and the size of the waste particles is reduced. The waste is dried under anaerobic conditions at a temperature range of 100 to 150° C., and partially pyrolyzed in a rotating, externally heated drum, at a temperature of between 140 and 400° C. The product is granulated using an extruder/mixer and the granules are carbonized under anaerobic conditions at a temperature in the range of 140 to 500° C. The carbonized granules are activated in the presence of steam and combustion gases of between 750 and 900° C. Finally, the activated granules are purified by rinsing in an aqueous hydrochloric acid solution, and subsequently drying the activated carbon.

U.S. Pat. No. 5,194,069 that issued to Someus discloses a method and an apparatus for the refinement of organic material. Converting and processing organic material is achieved with or without organic and inorganic additions. The base material uses animal or plant waste material, i.e. slaughter-house waste and forest industry waste. A slowly rotating waste container inside a furnace is used. Fuel for heating the cylindrical container is a mix of pyrolysis gases and purchased oil or natural gas. The reaction temperature is at 1650° F. (900° C.), and reduces the char to little more than carbon. The method and apparatus produces carbon powder/granulate as fuel, charcoal for grilling/smoking, activated carbon, or additives for steel production.

U.S. Pat. No. 5,017,269 that issued to Loomans et al. and U.S. Pat. No. 4,908,104 that issued to Loomans et al. disclose a method of continuously carbonizing a mixture of primarily organic waste material to a high calorific value char product. A stream of comminuted municipal waste material with a substantial organic material content is fed to one end of a mixer barrel. The material is compressed to form a barrel-filling mass functioning as a first vapor block, and the work energy required to compress the waste material and squeeze out entrapped air is used to raise the temperature of the material adiabatically. Air and any steam created are vented. The material downstream from the first vapor block is decompressed in a second vent region. The material is recompressed in the absence of air to form another vapor block, while exclusively utilizing the work energy required to compress it to raise the temperature of the material adiabatically to a volatile-releasing temperature in the neighborhood of 400 to 600° F. (204-316° C.), and to carbonize the material. The volatiles are vented, and the product is discharged as a dry, particulate char. The term “adiabatic heating” suggests that there is no heat transfer into the reaction chamber.

U.S. Pat. No. 4,098,649 that issued to Redker discloses an apparatus and method of converting organic material such as that separated from municipal and industrial waste into useful products by using a form of an extruder in a continuous destructive distillation process, and in which the material being processed is compressed in the extruder in the absence of air, and is heated to carefully controlled temperatures in separate zones to extract different products from each of the zones. Feedstock heating is performed by electric heating elements exterior to the reactor walls.

U.S. Pat. No. 3,787,292 that issued to Keappler describes a process and apparatus for the pyrolysis of solid wastes including a retort defining a plurality of interior temperature zones, a combustion tube disposed through the retort, a means for rotating the retort about the combustion tube, a waste infeed, a residue outlet, and at least one fluid exhaust communicating with the interior of the retort. The combustion tube is positioned down the center of what is essentially a rotary kiln, and is heated to 1500° F. (816° C.) Heat transfer is limited to the surface of the tube, and, the solids must be in contact with it to be heated.

The conversion process of the present invention focusses on the minimizing of the potential for dioxin formation, both in the conversion of waste or biomass into a high quality, dioxin-free solid fuel, and also in the utilization of that fuel product. The process maximizes end product uniformity, weight and energy yield, while carefully controlling processing temperature to effectively destroy dioxins present in waste or biomass, and tailoring the fuel product to further inhibit dioxin formation. The process's low reaction temperature effectively minimizes corrosion of reactor internal components, energy input and processing time. It utilizes a high capacity, continuous reactor to produce large quantities of a synthetic coal (char) consistent with the end use fuel market it is intended to serve.

It is an object of the present invention to provide an improved pyrolytic process for urban waste and biomass.

It is a further object of the invention to introduce an improved process for the preparation of a high quality synthetic coal (char) of uniform composition from urban waste and biomass.

It is a further object of the invention to destroy dioxins that may be present in urban waste and biomass.

It is a further object of the invention to create a synthetic coal (char) product that minimizes the potential for dioxin production during its utilization.

It is a further object of the invention to create a synthetic coal (char) product of uniform consistency, composition and quality from urban waste or biomass that may be of diverse and variable composition. It is a further object of the invention to create a synthetic coal (char) product that is directly compatible with existing coal combustion, storage, handling and pulverization systems.

It is a further object of the invention to more efficiently pyrolyze urban waste materials and biomass, to maximize the utilization of energy imparted to the reactor for pyrolysis.

It is a further object of the invention to introduce a pyrolysis system capable of continuous operation with high processing capacity, readily amenable to commercial scale large volume applications.

Other objects of the invention will become apparent as the description proceeds.

SUMMARY OF THE INVENTION

A process for the production of synthetic coal (char) from biomass or urban waste comprising sorting a material selected from the group consisting of i biomass, ii waste, and, iii a combination of biomass and waste, to remove foreign materials. Thereafter, shredding the sorted material and thereafter, pyrolyzing the sorted material at a temperature in the range of 500 to 600° F. in a pyrolysis reactor by means of in-situ viscous shear heating, and utilizing the liquid and gaseous byproducts of pyrolysis for the production of mechanical work used to cause the pyrolysis to occur.

Another embodiment of this invention is a process for the preparation of synthetic char coal from a material selected from the group consisting of i biomass, ii waste, and iii a combination of biomass and waste, comprising sorting the feedstock to remove foreign materials and reducing the size of the particles. Thereafter, pyrolyzing the non-foreign sorted material at a temperature in the range of 500 to 600° F. in a pyrolysis reactor, using viscous shear in-situ heating, to produce a granulated synthetic char coal having a moisture content of 3% or less. Collecting and cooling the granulated synthetic char coal and collecting and utilizing the byproduct oils and gases to produce mechanical energy to import into the pyrolysis reactor to accomplish said in-situ heating of the non-foreign sorted material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the temperature range for formation of PAH's.

FIG. 2 illustrates formation of dioxins from the PAH's in an oxidizing environment.

FIG. 3 illustrates the overall release rate for biomass pyrolysis products

FIG. 4 is a block flow diagram of the process for preparing synthetic coal from biomass or organic waste in the present invention.

FIG. 5 is a schematic diagram of the pyrolysis reactor and drive unit used in the conversion process for preparing synthetic coal from biomass or organic waste of this invention.

FIG. 6 is a cross sectional drawing of the elliptical mixing paddles and mixing chambers as implemented in the pyrolysis reactor of the present invention.

FIG. 7 is a reactor temperature profile showing the temperature difference between the material 16 in the reactor and the reactor wall 24 wherein the solid squares and heavy line represent the material temperature and the triangles and dashed line represent the reactor wall temperature.

FIG. 8 is a chart showing the chloride retention in char from this invention.

FIG. 9 is a chart illustrating the weight percent conversion to soluble chloride salts.

FIG. 10 is an illustration of the formation of coal.

FIG. 11 is a bar chart showing the fuel heating value of various materials.

FIG. 12 is a chart of product consistency from runs in the process of the instant invention.

FIG. 13 is a chart showing fuel sulfur content of coal and LTMP char.

FIG. 14 is a bar chart showing SO₂ emissions from various coals and LTMP char.

FIG. 15 is is a bar chart showing mercury content in coal and LTMP char.

FIG. 16 is a bar chart comparing CO₂ emissions from various coals and LTMP char of this invention evolving from 10,000 Btu/kWh heat rate.

FIG. 17 is a chart showing the burning profile analysis.

FIG. 18 is a bar chart showing boiler flame temperature.

FIG. 19 is a bar chart showing unburned hydrocarbons from various coals during the process of this invention.

FIG. 20 is a bar chart showing chlorine and sulfur retention by ash from the process.

FIG. 21 is a chart showing process conditions for formation and destruction of dioxins and their precursors.

FIG. 22 is a time/temperature profile.

FIG. 23 is a table of typical chlorinated hydrocarbons found in prior art waste destruction.

FIG. 24 is a table of typical PAH's and HAP's normally found in prior art waste destruction.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the following definitions are used. “In-situ” means that the heat is generated within the feedstock itself. “Organic waste” includes various types of waste produced in the urban environment. “Urban waste” includes domestic waste and commercial waste but may include industrial waste. “Domestic waste” includes waste produced in an average normal household which comprises food waste, paper products and packaging, plastic products, wood, glass and metal. “Commercial waste” is the waste produced by the commercial sector. Much of the commercial waste is generated by food establishments, markets, grocery stores and the like.

“Foreign materials” means materials that cannot be pyrolyzed and may interfere with the process, such as metal, glass, stones and ceramics. “Low Temperature Mechanical Pyrolysis” (LTMP) refers to the use of mechanical shaft work to cause the mixing and in-situ heating of biomass or urban waste as described in this present invention. Unless otherwise specified, all percentages are by weight, and all ratios between various process components are also by weight. This process is optimized such that the extent of pyrolysis is adjustable by configuration of the mixing and conveying elements within the reactor, allowing for the optimization of yields between the desired synthetic coal (char) and the remaining pyrolysis byproducts. In this process, using urban waste, the yield is set to approximately balance the energy content of the byproduct materials with the mechanical energy input needs of the reactor, including driver and other efficiency losses, minimizing the need for external fuel or other energy sources.

This process is further optimized to include a system for utilization of the byproduct gases and other heat sources emanating from the reactor, as energy sources to create the mechanical work input necessary to drive the pyrolysis reactions, in a highly efficient total system.

For a more complete understanding of the waste and biomass conversion process of the present invention, reference is made to the detailed description and accompanying drawings in which the presently preferred embodiment of the invention are shown by way of example. As the invention may be embodied in many forms without departing from the spirit of essential characteristics thereof, it is expressly understood that the drawings are for purposes of illustration and description only, and are not intended as a definition of the limits of the invention.

The process of the present invention addresses each of these objectives. According to the present invention, the organic fraction of urban waste is converted to a high quality synthetic coal (char) via an improved pyrolytic process. The synthetic coal (char) has a high calorific value, low moisture content, low sulfur content, and burning characteristics superior to naturally occurring high volatile bituminous coals.

The conversion process of the present invention takes advantage of in-situ heating, resulting from the conversion of mechanical work to heat through the shear forces of viscous mixing. The heat so produced is used to drive the chemical reactions of pyrolysis. Referring to FIG. 5, there is shown a schematic of a reactor of this invention. Within the reactor are three sequential mixing zones 9, 10, and 11. Each mixing zone is preceded by a zone of compaction i.e. A₁, B₁, and C₁ and is followed by a zone of expansion A₂, B₂, C₂. Compaction is used to completely fill the internal volume of the following mixing zone, eliminating air voids and ensuring that the waste can not migrate into void space to avoid the mixing process.

Referring to FIG. 6, mixing paddles 23 within the reactor mixing chamber 24 promote intense mixing of the feedstock, causing it to behave in a manner similar to that of a Bingham plastic, and creating intense viscous shear. Thus the mechanical energy input of the mixing paddles is converted directly into heat within the feedstock particles themselves rather than being conducted to the feedstock from hotter outside surfaces or circulating solid materials. As a result, there is (1) no reaction rate, capacity or size limitation due to heating surface, (2) no requirement for circulation of large quantities of hot sand, ceramics or other abrasive solids to transfer heat to the feedstock, (3) uniformity of heating throughout the feedstock, resulting in a product of uniform quality, (4) the ability to rapidly create large quantities of heat within the feedstock in a short period of time, resulting in a high capacity reactor, (5) the ability to achieve high levels of pyrolysis at lower reactor temperature due to the in-situ method of heat addition, making reactor materials selection simpler, (6) the ability to maximize pyrolysis performance at the temperatures best suited for destruction of dioxins, release and subsequent alkali capture of chlorine, and avoidance of formation of PAH's, (7) avoidance of the exterior wall heat loss problem resulting from reactor designs that use an external combustion chamber or heating element to heat waste contained in an internal reaction chamber, (8) avoidance of hot exterior wall surfaces that may promote the formation of sticky wall deposits, and (9) avoidance of combustion product heat losses to the stack in reactors that rely on transfer of heat from combustion products to the waste materials

Referring now to FIG. 4, the waste or biomass 16 is first sorted in a separator 1 for the removal of foreign materials 17. The feedstock is then shredded in a shredder 2 to a particle size of about 5 cm in diameter. The shredded feedstock material 20 is directly transferred to the mechanical feeder 8 at the drive end 3 of the pyrolysis reactor 4. The reactor 4 shown in detail in FIG. 5 is a high intensity horizontal mixer design, using multiple co-rotating mixing augers of overlapping design to transport the waste through the reactor 4 and to impart the mechanical work to the feedstock. Elliptical mixing paddles mounted on the augers 15 in each mixing zone 9, 10, and 11 stir and deform the feedstock, creating viscous shear heating throughout the feedstock within the mixing zone. The shapes of the paddles and mixing chamber provide limited clearance between reactor walls and the mixing paddles, no recessed areas where solids can avoid the mixing process, and a configuration whereby mixing paddles continuously wipe and clean each other and the mixing chamber walls.

Three mixing zones along the reactor volume deliver the work energy to the biomass or waste material within the reactor to accomplish drying and pyrolysis. Within these zones, the material is compacted to form a mass of moving material entirely filling the cross section of the reactor, to ensure that all mechanical energy of mixing is converted directly into in-situ heating within the feedstock itself. The compacted material also functions as a barrier to contain vapors within each mixing zone. The first zone 9 heats the feedstock to the range of 220 to 300° F. (104-150° C.), to evaporate moisture (water vapor 18) contained in the feedstock. The solids then pass to an expansion zone that allows water vapor to easily separate from the loose feedstock. The water vapor 18 so produced is removed through a vent 12 at the top of the reactor, immediately following the first mixing zone, carrying with it any air that may have been entrapped in the feedstock during initial compaction. This ensures that no free oxygen is available in subsequent mixing zones that might enable dioxin formation during pyrolysis. After recompaction, pyrolysis occurs in each of the following two mixing zones 10,11, with a relaxation region following each mixing zone which allows for the release of pyrolysis oils and gases 19 through vents 13,14 atop the reaction vessel. The material temperature in the pyrolysis zone increases to between 500 and 600° F. (260-316° C.). Referring to FIG. 7, temperature measurements of the reactor wall liner A and feedstock B during operation demonstrate that the heating process is indeed in-situ, as the material temperature exceeds that of the reactor walls.

Drying and pyrolysis combined take approximately 90 seconds. The final synthetic coal product 22 is transported by the mixing augers 15 to the discharge port at the end of the reactor vessel, where its temperature is reduced using a water-cooled screw conveyor 6. The cooling conveyor is closed to prevent atmospheric oxygen from reaching the hot synthetic coal as it exits the pyrolysis reactor, thus eliminating another potential source of dioxin formation.

To be effective in minimizing dioxin formation, the process in the present invention must first demonstrate that it is effective in stripping chlorine from existing dioxins and other chlorinated hydrocarbons within the virgin feedstock, and of sequestering chlorine in the form of alkali salts. The process of this invention is capable of stripping chlorine from the feedstock, liberating roughly 50% of it in the form of HC1, which is then captured from the reactor vent gases, and converted into harmless salt by reaction with an alkali sorbent. Referring to FIG. 8, the percentage of chloride liberation is relatively constant, even when the feedstock is salted with high doses of PVC plastic.

Next the process must demonstrate that the remaining chlorine that is not liberated and removed from the feedstock is tightly sequestered by the alkali salts. FIG. 9 shows that, even with salting of the feedstock to chloride levels up to 6% by weight, the chlorine remaining in the product char (synthetic coal) is almost completely bound as soluble chloride salts. In this figure, square and triangle data points represent LTMP Char and black ellipses represent the feedstock.

The completeness of chlorine liberation and sequestration is further verified by an analysis of the synthetic coal for chlorinated hydrocarbons. The process of the present invention has brought each of the chlorinated hydrocarbons listed in FIG. 23 to the level of undetectability, both in the synthetic coal product and also in the liquid by-products of the process.

The process has also achieved success in destroying or failing to produce polynuclear aromatic hydrocarbons and other hazardous air pollutants of concern. FIG. 24 lists 81 of these chemical species that were undetectable, either in the synthetic coal product or the liquid by-products of the process.

The process described in this invention substantially alters the chemical composition of the waste or biomass, following a path similar to that taken during the natural transformation of woody biomass to natural coal over millennia. Hydrogen and oxygen are stripped from the biomass molecules, resulting in a char product having an oxygen/carbon ratio virtually identical to that of coal. On the other hand, the ratio of hydrogen to carbon in the char is significantly higher than that of natural coal, indicating that combustion of the char will produce less CO2 when burned at the same energy input rate. FIG. 10 illustrates how this process parallels that of the natural coalification of biomass. However, this Low Temperature Mechanical Pyrolysis (LTMP) process is accomplished in a period of roughly 90 seconds. This is compared to the much more modest transformation that takes place in Torrefaction of biomass, which requires a reaction time nearly two orders of magnitude longer. Clearly the in-situ heating process used by this invention is far more effective than the external heating approach used in Torrefaction.

The synthetic coal (LTMP char) obtained from urban waste by the process of the present invention has a moisture content of 2% or less, a sulfur content of approximately 0.2%, and a calorific value of approximately 9,500-10,500 Btus/lb, giving it a calorific value comparable to midwestern bituminous coal, and far superior to those of most biomass and waste fuels as shown in FIG. 11 wherein the white bar represents the LTMP pyrolysis char from urban waste. Its calorific value is approximately double that of the urban waste that produced it.

The material is granular, and may be blended with natural coal for use in boilers and other combustion systems. Even though the urban waste feedstock is quite varied in composition, containing a mix of anything from tire chips to watermelon rinds, the chemical composition of the product char (synthetic coal) is remarkably uniform in composition, testifying to the effectiveness of the internal mixing process within the reactor. See FIG. 12 wherein samples 1, 2 and 3 were collected at the beginning, middle and end of an extended production run.

The product char is also very low in contaminants that significantly contribute to air pollution in coal fired power plants. For example, FIG. 13 compares the sulfur content of synthetic coal produced from urban waste by this process with that of natural coals as described in the U.S. Geological Survey coal data base wherein A is the U.S. Coal Reserves. The process of this invention removes a substantial portion of the sulfur that is present in the feedstock, while sequestering the majority of the remaining sulfur in the form of inorganic salts. As a result, the synthetic coal (char) is substantially lower in sulfur content than natural coals. This results in a significant reduction in SO₂ emissions when burned, compared to natural coals. See FIG. 14.

FIG. 15 provides a similar comparison of the mercury content of synthetic coal (char) produced from urban waste by this process, again using the USGS coal data base wherein A is the U.S. coal reserves. Once again, the LTMP char proves superior to the natural coal it displaces.

FIG. 16 demonstrates, by actual combustion testing, the premise of FIG. 10 in which a higher ratio of hydrogen to carbon in the synthetic coal should reduce the amount of greenhouse gas emissions resulting from the substitution of synthetic coal (LTMP char) for natural coal. Referring to FIG. 16, carbon dioxide measurements were compared, per unit of useful power output, between the LTMP char, Powder River Basin subbituminous coal and eastern bituminous coal. The combination of the LTMP char's lower moisture content and higher hydrogen content resulted in significantly lower CO2 emissions, compared to the natural coals, and specifically a 14.3% reduction when compared to Powder River Basin Coal.

To be truly effective at controlling or eliminating dioxin formation during combustion, the Low Temperature Mechanical Pyrolysis (LTMP) char product must demonstrate that it can ignite easily, burn rapidly and completely, to destroy PAH's and other unburned hydrocarbons that might later combine with available chlorine and oxygen to form de novo dioxins after combustion. Since coal-fired power plants produce less dioxins than other fuels and combustion processes, the LTMP char should be compared to coal. FIG. 17 compares the Burning Profile (TGA) curves for two samples of LTMP char against similar profiles for Powder River Basin subbituminous coal, Illinois bituminous coal and two other eastern bituminous coals. The LTMP char samples produce virtually identical burning profile curves, far to the left of those of the natural coals, indicating that (1) the LTMP mixing/in-situ heating method produces very consistent fuel products, (2) the char ignites sooner and easier than even the most reactive western coal, (3) the char burns much more rapidly than the natural coals, and (4) carbon burnout from the char is completed more rapidly than with the bituminous coals. This indicates that RAH destruction is at least as effective as that for bituminous coals. In this figure, A and B represent the two LTMP char samples, C is the PRB subbituminous coal, D is an Illinois #6 bituminous coal, E is a Pittsburgh #8 high volatile bituminous coal, and F is a Maryland medium volatile bituminous coal.

The U.S. Environmental Protection Agency has concluded that the high temperatures associated with combustion in coal fired power plants are largely responsible for the destruction of PAH's and the absence of TCDD (dioxin) in coal plant fly ash. FIG. 18 compares the adiabatic flame temperatures for municipal solid waste, woody biomass, five natural coals and LTMP char as produced from urban waste. The LTMP char product is capable of developing a flame temperature comparable to that of bituminous coals, and up to 1,000° F. hotter than those of wood biomass and municipal solid waste. This high temperature is sufficient to destroy PAH's and other dioxin precursors.

The success of LTMP char combustion in destroying PAH's and other hydrocarbons can be seen in comparative combustion tests against natural coals. FIG. 19 shows stack measurements of unburned hydrocarbons (reported as methane), as produced by an eastern bituminous coal blend, Powder River Basin subbituminous coal, and LTMP char as produced from urban waste. The combustion tests were carried out on a shaker grate stoker combustion unit, sequentially, changing no combustion parameters except for the fuel being fed. The less reactive bituminous coal resulted in unburned hydrocarbon emissions nearly 50 times greater than those produced by the more reactive subbituminous coal, while the LTMP char produced no measurable unburned hydrocarbons.

Further evidence of the LTMP char's ability to destroy PAH's and other precursors to de novo dioxin formation may be seen in the amount of unburned carbon remaining in the ash residue after combustion. In comparative combustion tests, the LTMP char ash is white in color, indicating the absence of unburned carbon, while the bituminous coal ash shows visible black carbon remaining. Clearly, LTMP char produces more complete carbon burnout than bituminous coal, and thus will have even less potential for de novo dioxin formation.

Further evidence of the efficacy of the process of this invention to control and minimize dioxin emissions may be seen in the ability of the alkali chloride salts within the LTMP char to sequester chlorine throughout the combustion process, to further inhibit the potential for de novo dioxin formation. FIG. 20 shows the degrees of sequestration of both sulfur and chlorine that were observed during comparative combustion tests of LTMP char, Powder River Basin subbituminous coal and an eastern bituminous coal blend. While all three fuels achieved some measure of sulfur sequestration, the LTMP char was more effective than coal by a factor of 3 to 6. Where chlorine sequestration was concerned, the performance was even more dramatic. The natural coals produced almost no chlorine sequestration at all, while the LTMP char retained nearly 60 percent of its chlorine content locked up in its ash.

The dioxin control methodology for this invention can best be explained by the curves and process paths described in FIG. 21. The direct combustion of biomass, or the incineration of urban waste, takes place in an oxidizing environment. As the combustion takes place, the temperature of the organic matter and combustion products increases to a level where PAH's and other dioxin precursors are readily formed. The relative rate of PAH formation reaches its maximum around 1700° F. (925° C.). However, during incineration, these biomass and waste fuels do not produce combustion temperatures that are sufficient to destroy the PAH's and other dioxin precursors. As a result, as the combustion products cool, they pass through the temperature range (around 650° F.) where de novo dioxin formation occurs. Here the necessary ingredients—PAH's, chlorine and free oxygen—are readily available.

The process of this invention, on the other hand, begins in a strongly reducing environment, and operates at a preferred temperature where chlorinated hydrocarbons and dioxins freely release their chlorine as HCl. In this same temperature and reducing environment, alkali within the ash of the waste or biomass preferentially capture the liberated chlorine, sequestering it. The processing temperature is controlled to prevent competition by ash silica from limiting the availability of alkali for chloride capture. As a result, the LTMP char that is produced is free of chlorinated hydrocarbons, and the remaining chloride is sequestered as alkali salts. When burned in a conventional combustor, the highly reactive LTMP char (synthetic coal) passes through the temperature range of PAH formation, rapidly achieving the temperatures that are needed for complete destruction of PAH's and dioxin precursors. Free chlorine availability is also reduced due to sequestration by the ash alkali. As a result, dioxin emissions are minimized.

The process temperature path and resulting dioxin control methodology are summarized in FIG. 22. Following the incineration path, combustion temperatures reach the zone where PAH's are readily formed, but alkali capture of chlorides is blocked by ash silica. The incineration process does not achieve temperatures sufficient to destroy the PAH's and other dioxin precursors. As the combustion products cool, they pass through the temperature zone where PAH's combine with chlorine and free oxygen to produce the de novo dioxins. These dioxins remain present at the lower final stack temperature. The process of this invention operates in two stages. In the strong reducing zone of LTMP pyrolysis, process temperatures are carefully controlled to maximize the destruction of chlorinated hydrocarbons, the liberation and alkali sequestration of chlorine. Later, the LTMP char is burned in a conventional coal-fired power plant, achieving flame temperatures sufficient to destroy PAH's and other precursors to de novo dioxin formation, while at the same time sequestering chlorine as alkali salts in the residual ash. As the combustion products cool through the de novo dioxin temperature range, dioxin formation is inhibited because the PAH's and dioxin precursors have been destroyed and most of the chlorine has been sequestered.

The following example is illustrative of a preferred embodiment of the invention, with reference to FIG. 4, which is a block diagram of the process. The following example is not to be construed as limiting, it being understood that a skilled person may carry out many obvious variations to the process.

EXAMPLE

Initially, 2000 pounds of urban waste are sorted to remove foreign material, FIG. 4, and shredded to produce approximately 1500 pounds of organic matter equivalent to Refuse Derived Fuel (RDF). The shredded waste material is fed by metered conveyor to the feed port 8 of the pyrolysis reactor, See FIG. 5, where it is conveyed and compacted by the internal reactor augers 15, which deliver it to the first mixing zone 9. Here intense mixing converts mechanical work into direct in-situ heating of the waste materials through viscous shear forces within the feedstock itself. During the short period in which the waste is maintained within the first mixing zone, the temperature of the waste is increased to approximately 260° F., liberating moisture in the form of water vapor. The waste leaves the mixing zone, passing into an area without compaction, which permits the vapors and solids to separate, with the water vapor leaving the reactor from a vent 12 on its top surface, at a temperature of approximately 260° F. (127° C.). Any air or free oxygen that may have been entrained in the feedstock during compaction is carried away by the vented water vapor, to ensure that it is not available during the subsequent pyrolysis stages of the process.

The waste is again compacted, and transported to the first of two pyrolysis zones. Again intense mixing by paddle elements of the augers 10 converts mechanical work into in-situ viscous heating, raising the temperature of the waste to approximately 500° F. (260° C.). After approximately 30 seconds, the partially pyrolized waste has passed through the second mixing zone, after which it again enters a zone without compaction. Here pyrolysis byproduct oils, gases and chlorine (as HCl) are liberated from the solids and leave the reactor through a vent port 13 on the top.

A third stage of compaction and mixing 11 occurs, raising the temperature of the waste materials to approximately 500 to 600° F. (260-316° C.), more fully pyrolyzing the waste materials. After, approximately 30 seconds of in-situ heating and pyrolysis, the material passes to a final uncompacted de-gassing zone, in which byproduct oils, vapors and chlorine (as HCl) are liberated. A vent port 14 on the top of the reactor allows for the removal of these byproduct materials. The remaining solids are conveyed to the discharge end of the mixing reactor, where they are then delivered to an enclosed contact cooler, consisting of a water cooled conveying screw 6. This cooling system reduces the temperature of the synthetic coal (char) product 22 in an oxygen-free environment, without direct contact with process water, stopping the pyrolysis reaction, and preventing any post-pyrolysis dioxin formation. Heat recovered from the cooling system is utilized for boiler feedwater heating in the preferred embodiment of this process.

The solid product of pyrolysis, weighing about 830 pounds, is similar in appearance to granular coal, has a moisture content of less than 2%, a calorific value of approximately 9,500-10,500 Btus/pound, and a sulfur content of approximately 0.24%. It is easily ignitable, and exhibits a burning profile similar to, but more reactive than, natural subbituminous coal. The material may be transported, stored and pulverized in a manner similar to natural coal, and may be blended with natural coal for boiler and other combustion applications.

In the preferred embodiment of this process, byproduct oils and gases from the two reactor pyrolysis zone vents 13,14 are conveyed in vapor phase, without cooling, directly into a refractory-lined combustion chamber, where they serve as the primary fuel source. Water vapor from the reactor drying zone vent 12 is combined with combustion air entering this same chamber. A small amount of pilot fuel is used to start the fuel ignition process and to balance the energy needs of the process, as required. For urban waste, however, the process is optimized such that the energy content of the vent gases and oils closely approximates what is needed to drive the pyrolysis reactor, when all process and drive inefficiencies are accounted for.

The vent gas combustor operates at a temperature in excess of 2,700° F. (1,480° C.), for a residence time in excess of 1 second, to ensure complete combustion and destruction of any PAH's that may have formed early in the vent gas combustion process and could have later formed de novo dioxins. Combustion products enter a boiler system 5 optimized to maximize the energy recoverable from the superheated steam 21, while minimizing the risk of high temperature acid gas corrosion. An alkali scrubber system is employed to capture chlorine (HCl) and sulfur (SO₂) in the combustion gases, converting them to harmless salts, and further minimizing the possibility of dioxin formation. The steam is directed to a condensing steam turbine generator set, which delivers electric energy to the pyrolysis reactor drive motor 3 (See FIG. 4). In an alternative, a steam turbine may be used to directly drive the mixing augers in the pyrolysis reactor.

FIG. 23 is a table of chlorinated hydrocarbons that are normally found in prior art waste destruction. None of these materials were detected in either the LTMP char or bio-oil produced by the inventive process herein. FIG. 24 is a table of PAH's and other HAP's that are normally found in prior art waste destruction. None of these materials were detected in either the LIMP char or bio oil produced by the inventive process herein.

It is evident that many alternatives, modifications, and variations of the biomass and waste conversion process of the present invention will be apparent to those skilled in the art in light of the disclosure herein. It is intended that the metes and bounds of the present invention be determined by the appended claims rather than by the language of the above specification, and that all such alternatives, modifications, and variations which form a conjointly cooperative equivalent are intended to be included within the spirit and scope of these claims. 

What is claimed is:
 1. A process for the production of synthetic coal (char) from biomass or urban waste comprising: (a) sorting a material selected from the group consisting of i biomass, ii waste, and iii a combination of biomass and waste, to remove foreign materials; (b) shredding said sorted material; (c) pyrolyzing said sorted material at a temperature in the range of 500 to 600° F. by means of in-situ viscous shear heating; and (d) utilizing the liquid and gaseous byproducts of pyrolysis for the production of mechanical work used to cause said pyrolysis to occur.
 2. The process according to claim 1, whereby said sorted material is both dried and pyrolized within a single reactor.
 3. The process according to claim 1, whereby said pyrolysis is conducted in a twin screw mixer, converting the mechanical shaft energy of mixing using viscous shear forces to produce in-situ heating of the sorted materials.
 4. The process according to claim 1, whereby unique mixing using a viscous shear heating method, combined with the process temperature range, maximizes the destruction of chlorinated hydrocarbons, including dioxins, within the sorted material.
 5. The process according to claim 1, whereby a unique heating method and temperature range combine to maximize the liberation and alkali sequestration of chlorine to minimize future formation of dioxins during combustion of the char product of said process.
 6. The process according to claim 1, whereby a unique heating method and temperature range combine to maximize the energy efficiency of sorted materials pyrolysis.
 7. The process according to claim 1, whereby the sorted materials are reduced to a uniform, hard granular solid fuel product by means of in-situ heating and mixing caused by said reactor.
 8. The process according to claim 1, whereby a solid fuel product of said process has significantly lower potential for de novo dioxin formation than its sorted material, due to product high reactivity and high combustion temperature.
 9. The process according to claim 1, whereby the solid fuel product of said reactor has significantly lower potential for de novo dioxin formation, due to the sequestration of chlorine by alkali salts present in any product ash.
 10. The process according to claim 1, whereby said solid fuel product of said reactor has a calorific value in the range of 9,000-10,500 Btu/pound.
 11. The process according to claim 1, whereby said solid fuel product of said reaction reactor is similar in handling, storage and burning characteristics to high volatile bituminous coal.
 12. The process according to claim 1, whereby said solid fuel product of this reactor is capable of reducing greenhouse gas emissions from combustion sources, compared to natural coal.
 13. The process according to claim 1, whereby said pyrolysis is conducted in a temperature range of 500 to 600° F.
 14. The process according to claim 1, whereby byproduct water vapor, oils and gases are removed from said pyrolysis reactor by separate vents.
 15. The process according to claim 1, whereby byproduct water vapor produced in a first mixing/drying zone of said reactor is used to remove any entrapped air from said sorted materials by way of a first gas vent, thereby reducing dioxin formation potential in subsequent pyrolysis zones in said reactor.
 16. The process according to claim 1, whereby byproduct oils, gases and sensible heat are utilized in a boiler to produce steam.
 17. A process according to claim 1, whereby the energy requirements of pyrolysis are met by the introduction of mechanical energy, which is converted to in-situ heating of the sorted materials within said reactor by the process of viscous shear during mixing.
 18. The process according to claim 1, whereby the use of in-situ heating within and throughout said reactor provides for rapid heating and conversion of said sorted materials without any limitations in reaction rate imposed by transfer of heat through reactor walls and other surfaces.
 19. A process according to claim 1, whereby the use of in-situ heating within and throughout said reactor provides for conversion of the sorted materials without any limitations in product uniformity imposed by delivery of heat by conduction through contact with reactor wall surfaces.
 20. A process according to claim 1, whereby the use of in-situ heating within and throughout the reactor provides for the scaling of said reactor capacity based on said reactor mixing zone volume rather than area of heated surface.
 21. A process according to claim 1, whereby the effective heating of sorted materials for said pyrolysis can be accomplished without risk of any ash softening and deposition on reactor walls and other surfaces.
 22. A process for the preparation of synthetic char coal fuel from a feedstock selected from the group consisting of i biomass, ii waste, and, iii a combination of biomass and waste, comprising: (a) sorting said feedstock to remove foreign materials; (b) reducing the size of non-foreign feedstock; (c) pyrolyzing said feedstock at a temperature in the range of 500 to 600° F., using viscous shear in-situ heating, to produce a granulated synthetic coal char fuel having a moisture content of 3% or less; (d) collecting and cooling said granulated synthetic coal char fuel, and, (e) collecting and utilizing the byproduct oils and gases to produce mechanical energy to import into the pyrolysis reactor to accomplish in-situ heating of the sorted non-foreign materials. 